Seeing the Viessmann boiler F1 fault code appear on your Vitodens combi or system boiler display can be alarming, especially in the middle of winter. The F1 fault code is one of the most common error messages on Viessmann domestic boilers in the UK, and it nearly always points to the boiler overheating and going into safety lockout.
In this guide, I’ll explain exactly what the F1 fault means, the real-world causes, and safe steps you can take right now.
What Does the Viessmann Boiler F1 Fault Code Mean?
The Viessmann boiler F1 fault code means the burner has shut down because the maximum permitted flue gas temperature or boiler flow temperature has been exceeded. In simple terms: your boiler has got too hot and automatically locked itself out to protect the heat exchanger and prevent damage. If you encounter the Viessmann boiler F1 fault code, it’s essential to allow the system to cool down before resetting it. Regular maintenance and checks on the flue and radiator systems can help prevent this issue from occurring again. Additionally, for a deeper understanding of related issues, the Viessmann boiler F2 fault code explained can provide insights into different operational concerns that might arise. If the F1 fault code persists even after cooling down and resetting, it may indicate a more complex issue that requires professional attention. It’s also beneficial to familiarize yourself with other fault codes, such as the “viessmann boiler f5 fault code explained,” to gain a clearer perspective on troubleshooting potential problems. Taking proactive steps, like scheduling regular services, can ensure your boiler operates efficiently and safely. Being aware of common Viessmann boiler issues can significantly reduce the likelihood of encountering fault codes like F1 in the future. Regular inspections can identify potential problems before they escalate, providing peace of mind and maintaining the efficiency of your heating system. Additionally, consulting a qualified technician can offer tailored solutions and advice for your specific model, ensuring longevity and optimal performance.
This code appears on popular models such as the Vitodens 050-W, 100-W, 111-W, 200-W and 222-F series.
What Causes the Viessmann F1 Fault Code?
The F1 fault is nearly always triggered by poor water circulation or restricted heat transfer inside the boiler. Here are the proven causes we see every week across UK homes and rental properties.
Cause 1: Low System Water Pressure
Low pressure (usually below 0.8–1.0 bar) is the number-one reason for F1 on Viessmann combi boilers.
How to fix low pressure causing F1
- Locate the filling loop – usually a silver braided hose or two blue/grey handles underneath the boiler.
- Open both valves slowly until the pressure gauge rises to 1.3–1.5 bar (green zone on most Viessmann models).
- Close the valves tightly and remove any key if used.
- Press the reset button (usually marked with an R or flame symbol) for 3 seconds. The F1 should clear immediately if pressure was the only issue.
Cause 2: Sludge, Limescale or Airlocks in the System
Black iron oxide sludge, limescale (especially in hard-water areas like London and the South East), or trapped air stops water circulating properly, causing hot spots and F1 lockouts.
How to fix sludge/air causing F1
- Bleed every radiator starting downstairs and working up, then top up pressure again.
- If the fault returns within hours or days, the system needs a professional powerflush and inhibitor refill.
- Fit a magnetic system filter (such as the Viessmann/Viessmann-approved MagnaClean) to prevent recurrence.
Cause 3: Circulating Pump Failure or Incorrect Speed Setting
The pump pushes hot water around the system. If it seizes, runs too slowly, or is set to the wrong speed, circulation drops and F1 appears.
How to fix pump issues causing F1
- Listen for a quiet humming from inside the boiler when heating is on – no noise usually means the pump has failed.
- On models with adjustable pumps (Vitodens 100-W & 111-W), an engineer can increase the speed from setting II to III.
- Replacement Viessmann Grundfos or Wilo pumps cost £180–£280 fitted.
Cause 4: Blocked or Partially Blocked Plate Heat Exchanger (Combi Models Only)
On combi boilers, the secondary plate heat exchanger that heats tap water can fur up with limescale, restricting flow when the hot tap is run and pushing the primary side into overheat.
How to fix a blocked plate heat exchanger
- In hard-water areas, descaling the plate every 3–5 years prevents this.
- A Gas Safe engineer removes and chemically cleans or replaces the plate (typical cost £220–£350).
Cause 5: Faulty Flow or Boiler Temperature Sensor (NTC)
The flow NTC sensor sits on the heating pipe leaving the boiler. If it drifts out of calibration or fails, the control unit thinks the boiler is far hotter than it actually is and triggers F1.
How to fix a faulty NTC sensor
- This requires diagnostic equipment to test resistance.
- Viessmann flow NTC sensors are inexpensive (£25–£40) and quick to swap – a common same-day repair.
Cause 6: Diverter Valve Stuck in Hot-Water Position (Combi Boilers)
When the motorised diverter valve jams halfway, the boiler tries to heat both heating and hot water at once, leading to overheating and F1.
How to fix a stuck diverter valve
- Engineers free the valve or fit a new cartridge/motor head – usually £180–£260 including labour.
How to Reset a Viessmann Boiler Showing F1
- Wait 5–10 minutes for the boiler to cool slightly.
- Press and hold the reset button (R or flame icon) for about 3 seconds until the display changes.
- The boiler will attempt to restart – watch the pressure gauge and listen for the pump. Only reset twice. If F1 returns immediately, stop and call a Viessmann-trained or Gas Safe engineer.
FAQs
Why does my Viessmann boiler show F1 only when I run a hot tap?
This almost always points to a scaled-up plate heat exchanger or diverter valve problem on combi models – the extra demand causes instant overheating.
Will my Viessmann extended warranty still be valid if I top up the pressure myself?
Yes – repressurising using the filling loop is considered normal user maintenance and does not affect the 5–12 year Viessmann warranty.
Can outside temperature affect the F1 fault appearing more often?
Yes, in very cold weather the pump works harder and any existing sludge or low pressure becomes critical, triggering F1 more frequently.
How quickly should a landlord respond to a tenant reporting F1?
Treat it as urgent – no heating or hot water in winter is classed as an emergency repair under UK landlord legislation. Arrange an engineer within 24 hours.
Is the F1 fault code stored in the boiler’s fault history?
Yes, Viessmann boilers save the last 10 faults. Press the “i” info button and scroll using the dial – very useful for the engineer to see how often it has happened.
The Boilers2Go Team
With years of experience in boilers and home improvements in general, Myles Robinson is a passionate advocate for making home heating simpler, safer, and more affordable for households across the UK. With years of experience in the heating industry, Myles combines hands-on knowledge with a focus on customer-first service, helping homeowners choose the right boiler, understand their systems, and keep their heating running efficiently all year round. His articles break down complex topics into clear, practical advice you can trust.